 
 (defn rember [a lat]
   (cond
-    (null? lat) '()
+    (null? lat) []
     (= (first lat) a) (rest lat)
     :else (cons (first lat)
                 (rember a (rest lat)))))
 
 (defn firsts [l]
   (cond
-    (null? l) '()
+    (null? l) []
     :else (cons (first (first l))
                 (firsts (rest l)))))
 
 (defn insertR [n o lat]
   (cond
-    (null? lat) '()
+    (null? lat) []
     (= (first lat) o) (cons o (cons n (rest lat)))
     :else (cons (first lat)
                 (insertR n o (rest lat)))))
 
 (defn insertL [n o lat]
   (cond
-    (null? lat) '()
+    (null? lat) []
     (= (first lat) o) (cons n lat)
     :else (cons (first lat)
                 (insertL n o (rest lat)))))
 
 (defn subst [n o lat]
   (cond
-    (null? lat) '()
+    (null? lat) []
     (= (first lat) o) (cons n (rest lat))
     :else (cons (first lat)
                 (subst n o (rest lat)))))


(defn multirember [a lat]
  (cond
    (null? lat) '()
    (= (first lat) a) (multirember a (rest lat))
    :else (cons (first lat)
                (multirember a (rest lat)))))

(defn multiinsertR [n o lat]
  (cond
    (null? lat) '()
    (= (first lat) o) (cons o (cons n
                                    (multiinsertR n o (rest lat))))
    :else (cons (first lat)

; end of LISPer, bonus practice
(defn subst2 [n o1 o2 lat]
  (cond
    (null? lat) []
    (or (= (first lat) o1)
        (= (first lat) o2)) (cons n (rest lat))
    :else (cons (first lat)
